Mention that additional args can be passed to eg. mysql

Which is very useful, for example when trying to directly run SQL statements via mysql.

AFAIS this currently affects mysql, mysql-replica and redis.

This will change to --help output from

[…]
Usage:
  mw docker mysql mysql [flags]
[…]

to

[…]
Usage:
  mw docker mysql mysql [flags] -- [mysql flags]
[…]